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Phytoplankton: "Grass of the Seas"
From: Columbia University
| By:
Ray Sambrotto |
EDITOR'S INTRODUCTION |
 Roughly half of the Earth's biological production stems from microscopic, one-celled plants called phytoplankton that grow in the surface layers of the ocean. Phytoplankton's impact extends far beyond its microscopic scale--it forms the base of the marine food chain and may play a particularly critical role in reducing atmospheric carbon dioxide (CO2). Ray Sambrotto, a research scientist at Columbia University's Lamont-Doherty Earth Observatory, studies patterns of phytoplankton productivity and their effect on the Earth's climate. |
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| The ocean's biological carbon pump. | |
 nce called the "grass of the seas," phytoplankton ultimately provide food for almost all other life in the ocean. From the fish swimming near the surface to some of the most unusual organisms inhabiting the ocean floor, they all depend in some way on the growth of phytoplankton. Recent scientific research suggests that phytoplankton also play a critical role in maintaining the Earth's environment by removing greenhouse gases like CO2 from the atmosphere. Each year, phytoplankton fix 50 petagrams (1015 grams) of carbon into plant material. They absorb much of that carbon from the atmosphere and transport some of it to deeper waters as the organic matter sinks. This vertical transport, called the ocean's "biological carbon pump," helps to reduce CO2 levels in the atmosphere. Such levels have been increasing because of the burning of fossil fuels, and they threaten to cause a warming of the Earth's climate. |
Phytoplankton research
Research on phytoplankton biology has two major applications:
- The analysis of marine food chains.
- The role of marine production in the Earth's climate system.
As a modern example of the former, the fishing industry tracks phytoplankton because they are a major ocean food source. Pinpointing the regions of greatest phytoplankton growth helps to locate the fish that congregate in these productive regions to feed. |
One of Earth's oldest and most productive biological systems, phytoplankton have long been a subject of basic research. In the early part of the twentieth century, scientists conducted phytoplankton research by going out on a ship, putting a bucket over the side, filling it up, pulling it back onto the deck and using a microscope to analyze the number and species of phytoplankton in a given sample. Scientists applied this classical, naturalistic approach to all of marine biology, studying the abundance and taxonomy of organisms. For selected regions of the ocean, we have records of such studies on phytoplankton extending back more than 100 years. Later, scientists began to investigate the interaction between ocean chemistry and fertility. This link provides a climatic role for phytoplankton--in particular because of their consumption of atmospheric CO2. |
Phytoplankton biology and productivity
In essence, phytoplankton are not too different from land plants. Both make their living through photosynthesis and both interact with the atmosphere, taking up CO2 and giving off oxygen. The most obvious difference appears in their relative sizes. Sequoias and other trees grow to great heights in order to compete for sunlight. Phytoplankton remain microscopic because increased size does not provide them with any evolutionary advantage. Since the bottom of the ocean may reach down 4,000 meters--far from any visible light--phytoplankton's engineering solution is to stay small and share the surface water. |
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| Ceratium longipes, a type of dinoflagellate. Some members of this phytoplankton group are toxic to vertebrates. | |
On a microscopic scale, the phytoplankton community is composed of very diverse taxa in which ancient species exist next to recent arrivals; the relatively stable environment provided by the ocean over the millennia has enabled this biological diversity. In contrast, we see on land mainly the latest round of evolutionary winners. Extreme phenomena like ice ages regularly alter the terrestrial environment, destroying many older plant forms that cannot adapt. |
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| Pyramimonas disomata, a small flagellated form of phytoplankton that is widely distributed. | |
Phytoplankton do not grow in equal amounts throughout the ocean. Recognizing variations in plant productivity on land is second nature; a child can tell the difference between a productive forest, a grassland and a desert. While less apparent, substantial variations in biological productivity also exist within the ocean. Areas with abundant phytoplankton may contain approximately 10-15 micrograms of phytoplankton chlorophyll per liter, versus 0.1 micrograms in unproductive regions. |
What causes these large variations? All plant life requires the same three basic elements to survive:
- Water.
- Light.
- Nutrients.
Water is obviously not a problem for phytoplankton. They also have enough light as long as they can maintain themselves near the surface during the growing season. However, the availability of nutrients is critical for phytoplankton growth. |
Although we have talked a lot about how phytoplankton growth affects CO2, plants cannot make organic matter out of carbon alone; they need a variety of other nutrients. For example, when one of your houseplants is not looking well, you might use a product such as Miracle-Gro, which contains several nutrient salts--including nitrate, phosphate, sulphur and iron--to nurse it back to health. All plants need these nutrients to build organic matter; finding them all in the same place and at the same time turns out to be a limiting factor for the growth of marine phytoplankton. |
The ocean's biological carbon pump <br>and atmospheric CO<sub>2</sub>
Just like leaves falling from trees, at some point the plant material created by phytoplankton falls from the surface of the ocean. Often, this rain of organic material from the surface of the ocean is accelerated by the feeding of other organisms on phytoplankton. Once the organic matter reaches more than a few hundred meters below the ocean's surface, it becomes isolated from the atmosphere--protected by a blanket of water. The net effect is to pump organic material--including atmospheric CO2--away from the surface. Scientists refer to this process--pulling CO2 out of the atmosphere, fixing it into organic material in the surface water and ultimately transporting it down into the ocean depths--as the ocean's biological carbon pump. |
One of the important effects of the ocean's biological carbon pump is to remove atmospheric CO2. This is significant because CO2 is one of the greenhouse gases that trap the Earth's radiation and warm our atmosphere. The concern is that an increase in such gases will result in unknown (and perhaps unwanted) climate changes. For more than 100 years, people have kept careful financial records of fossil fuel (coal and oil) consumption; by examining these records we can determine levels of human CO2 production. Currently, we produce about six gigatons of CO2 from the burning of fossil fuel each year. Yet, when we look for the corresponding increase in atmospheric CO2, we find only about four of these six gigatons. |
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| Chain of Chaetoceros decipiens from the North Atlantic. The inset shows the individual chloroplasts of several cells--where photosynthesis takes place. | |
Where do the other two gigatons of carbon go? At present, this remains a research question, but it is clear that the ocean absorbs a significant amount of the CO2 created by the burning of fossil fuels. We think that much of this is a result of the ocean's biological carbon pump. To determine how much CO2 is pumped to the deep ocean, we need to know not only the amount of phytoplankton production but also the efficiency with which organic matter is transported to the ocean depths. Both aspects of the pump are complex and need to be addressed, not only to determine how much carbon is being absorbed at present but also to predict how the pump's role may change in the future. |
By deploying sediment traps at various ocean depths to collect the rain of particles as they fall, we know that most of the organic matter produced in surface water is destroyed before reaching the seafloor. This material may get eaten by larger organisms in deeper waters or may be decomposed by bacteria. However, even though little of the phytoplankton production is preserved for long periods in ocean sediments, its movement to deeper waters keeps it out of the atmosphere for hundreds of years. |
Although land plants fix a similar amount of carbon into structural materials like wood and leaves each year, these materials remain exposed to the atmosphere. When the plants die, the CO2 and other elements they contain return to the atmosphere. Unlike plant material on land, however, when the phytoplankton die, get eaten or otherwise leave the surface layer, they can fall thousands of meters to the bottom of the ocean. Thus the ocean's biological pump may buy us more time to determine the climatic impact of increased CO2 levels and to do something about the process. |
Do nutrients limit the pumping rate?
The other aspect controlling the efficiency of the ocean's biological carbon pump to absorb CO2 is the amount of phytoplankton production. Large regions of the ocean--probably the majority of the ocean's surface--have been completely stripped of nutrients by years of phytoplankton growth. The tropics, for example, have crystal-clear turquoise water because the low levels of nutrients in these waters severely limit the growth of phytoplankton. Deep water provides the main source of ocean nutrients, recycling materials from the ocean carbon pump. While the pump pushes most of the material down to the ocean depths, some returns to the surface layer. |
With the advent of satellite remote sensing in the past 20 years, we now have access to a synoptic or global view of the Earth's systems. To study phytoplankton, remote sensors use specific wavelengths (bands) of visible light to estimate levels of chlorophyll--a pigment common to all higher plants--in a given section of ocean. These images provide a planet-wide perspective on patterns of phytoplankton growth. |
Remote sensors have confirmed earlier ships' observations of a consistent pattern of decreased phytoplankton productivity in low latitudes and increased productivity in high latitudes.
Low latitudes such as the tropics have a very stratified system of warm, salty water on the surface and colder water down deep. Like oil and water, these two different density layers do not mix well, preventing nutrients from being resupplied back to the surface. High latitudes show much greater levels of productivity because of increased mixing between ocean layers. In particular, high-latitude surface waters receive a fresh batch of nutrients each winter when cooling temperatures allow the surface and deep waters to mix. |
Coastal and upwelling regions also show greater phytoplankton productivity. Coastal water gets nutrients by vigorous mixing on continental shelves and from rivers. Upwelling refers to an unusual process in which deeper, nutrient-rich water is pulled to the surface, overcoming surface stratification. An abundance of phytoplankton exists along the equator because of upwelling in this region. |
Investigating the role of micronutrients
But the availability of major nutrients alone does not fully explain global patterns of phytoplankton productivity; low levels of phytoplankton are often found in areas of the ocean that contain plenty of nutrients like nitrate and phosphate. A theory has emerged that these areas may lack certain micronutrients--in particular, metals such as iron--that have not been measured extensively. |
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| Phaeocystis pouchetii, a colonian form common in polar regions. | |
In terrestrial systems, metals are relatively abundant in most soils. Once you move away from coastal regions, however, the ocean often has vanishingly small levels of iron and other metals. Where does iron come from in the ocean? One possible source is soil dust blown out to sea. If true, then patterns of phytoplankton productivity should mirror those of dust-carrying, offshore winds. This correlation holds in some regions--for example, the North Atlantic west of the Sahara--although it is less clear in others. |
Phytoplankton's global impact
The transport of micronutrients to the ocean from the land represents just one more piece in the phytoplankton puzzle--raising as many questions as it answers. For example, scientists have determined from sediment and ice cores that when the Earth gets colder, it also gets drier and dustier. If the previous theory holds, then increased dust may mean an increased input of iron into the ocean. Greater levels of iron may in turn spur greater phytoplankton productivity. Increased phytoplankton populations pull more CO2 out of the atmosphere, and that in turn would further lower atmospheric temperatures. |
While this chain of complex interactions is purely hypothetical, it demonstrates just one possible scenario of biological climate feedback. In reality, we remain unsure of the cause-and-effect nature of this relationship. Does atmospheric CO2 drop (and climate cool) because ocean fertility increases, or does ocean fertility dance to the tune called out by climate change? |
This is an exciting question, because its answer requires a detailed understanding of the processes controlling the climate of our planet. Whatever the ultimate answers may be, the ocean's production system clearly plays an important role. For example, consider the Earth with a sterile ocean surface, containing no phytoplankton growth. (Such an experiment can be run mathematically using global climate models, or GCM, computer models that represent the physical, chemical and biological interactions of the Earth's systems.) While exact results vary according to the specific GCM used, stripping out phytoplankton easily doubles the amount of atmospheric CO2 within decades. This scenario further demonstrates the powerful impact of microscopic phytoplankton on the planet. |
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